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Showing content from https://github.com/dotnet/extensions/issues/2149 below:

Make BackgroundService startup not block host startup · Issue #36063 · dotnet/runtime · GitHub

IHostedService.StartAsync executes inline (and sequentially) when IHost.Start/StartAsync is called. With a BackgroundService, this causes lots of confusion since running code that isn't async in ExecuteAsync or StartAsync also runs inline. The workaround today is to call Task.Run on your own code or to use Task.Yield(), but this is not obvious.

public class BackgroundService : IHostedService
{
+    public virtual bool StartAsynchronously { get; } = true;
}

NOTE: StartAsynchronously would be true by default.

class MyBackgroundService : BackgroundService
{
    // It's true by default, this is just for illustration.
    public override bool StartAsynchronously => true;

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        using var timer = new PeriodicTimer(TimeSpan.FromSeconds(5));

        while (await timer.WaitForNextTickAsync(stoppingToken))
        {
            Console.WriteLine("Hello from background service!");
        }
    }
}

An optional constructor argument.

BackgroundService blocked the execution of whole host

When I run the specific foreach cycle in BackgroundService-derived class, it blocks the whole host from starting. Even second hosted service doesn't start. When I comment foreach cycle, everything works as expected.

TargetFramework: netcoreapp2.1
Version: 2.1.12

Use following hosted service: NotificationRunner.cs
And singleton: NotificationService.cs

BackgroundService.ExecuteAsync should work in background without blocking even if it has blocking code. As you can see in NotificationService class, cancellation of enumerator is based on IApplicationLifetime.ApplicationStopping, but anyway it shouldn't affect the host startup because BackgroundService is expected to run in background :)

When foreach cycle exists

Then execution is blocked on this cycle

But when foreach cycle is commented

Then execution continues as expected

(But why twice?)

.NET Core SDK (reflecting any global.json):
 Version:   3.0.100-preview7-012821
 Commit:    6348f1068a

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.17763
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\3.0.100-preview7-012821\

Host (useful for support):
  Version: 3.0.0-preview7-27912-14
  Commit:  4da6ee6450

.NET Core SDKs installed:
  2.1.700 [C:\Program Files\dotnet\sdk]
  2.1.701 [C:\Program Files\dotnet\sdk]
  2.1.801 [C:\Program Files\dotnet\sdk]
  2.2.300 [C:\Program Files\dotnet\sdk]
  2.2.301 [C:\Program Files\dotnet\sdk]
  2.2.401 [C:\Program Files\dotnet\sdk]
  3.0.100-preview7-012821 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.0.0-preview7.19365.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.0.0-preview7-27912-14 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.0.0-preview7-27912-14 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

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